In the course of routine analyses of the free amino acids in human blood plasma several quantitatively minor unidentified ninhydrinpositive materials have been observed consistently. It is proposed to isolate and identify some of these substances and to determine their manner of origin and possible physiological significance. An investigation on the nature of the bound amino acids in deproteinized blood plasma will also be made. Deproteinized bovine blood plasma will be used as a source of the unidentified materials; it has been demonstrated to have a similar content of these substances, qualitatively and quantitatively, as human plasma. Ion-exchange and thin-layer chromatography and high voltage electrophoresis will be used for separation and purification of the compounds.